MXPA06007875A - Frequency error estimation and frame synchronization in an ofdm system - Google Patents

Abstract

Frequency error estimation and frame synchronization are performed at a receiver in an OFDM system based on a metric that is indicative of detected pilot power. The metric may be defined based on cross-correlation between two received symbols obtained in two OFDM symbol periods. For frequency error estimation, a metric value is computed for each of multiple hypothesized frequency errors. The hypothesized frequency error for the metric value with the largest magnitude is provided as the estimated frequency error. For frame synchronization, a correlation value is obtained for each OFDM symbol period by correlating metric values obtained for NC (e.g., most recent) OFDM symbol periods with NC expected values. The expected values are computed in a manner consistent with the manner in which the metric values are computed. Peak detection is performed on the correlation values obtained for different OFDM symbol periods to determine frame synchronization.

Description

ESTIMATION OF FREQUENCY ERROR AND SYNCHRONIZATION OF FRAME IN AN OFDM SYSTEM FIELD OF THE INVENTION The present invention relates generally to data communications, and very specifically to techniques for executing frequency error estimation and frame synchronization in an orthogonal frequency division multiplexing (OFDM) communication system.

BACKGROUND OF THE INVENTION OFDM is a multi-carrier modulation technique that has the ability to provide high performance for some wireless environments. OFDM effectively divides the bandwidth of the global system into multiple orthogonal sub-bands (Nsb), which are commonly referred to as tones, sub-carriers, deposits and frequency channels. With OFDM, each subband is associated with a respective sub-carrier that can be modulated with data. In an OFDM system, initially a transmitter encodes, intercalates, and modulates a stream of information bits to obtain a stream of symbols from modulation. In each period of OFDM symbols, Nsb "transmission" symbols may be sent in the Nsb subbands, where each transmission symbol may be a data symbol (ie, a modulation symbol for data), a pilot symbol (that is, a modulation symbol for pilot), or a signal value of zero. The transmitter transforms the Nsb transmission symbols into the time domain using a fast inverse Fourier transform (IFFT) and obtains a "transformed" symbol containing Nsb time domain chips. To combat selective frequency fading (ie, a frequency response that varies in the Nsb subbands), which is caused by a multiple path in a wireless channel, a portion of each transformed symbol is typically repeated. The repeated portion is often referred to as a cyclic prefix and includes cp chips. An OFDM symbol is formed by the transformed symbol and its cyclic prefix. Each OFDM symbol contains NL chips (where NL = Ns + Ncp) and has a duration of NL chip periods, which is an OFDM symbol period (or simply "symbol period"). The transmitter can transmit the OFDM symbols in frames, where each frame contains multiple OFDM symbols (Nsym). The frames of the OFDM symbols are further processed and transmitted to a receiver.

The receiver executes the complementary processing and obtains NL samples for each received OFDM symbol. The receiver removes the cyclic prefix of each received OFDM symbol to obtain a received transformed symbol. The receiver then transforms each received transformed symbol to the frequency domain using a Fast Fourier Transform (FFT) and obtains Nsb "received" symbols for the Nsb subbands, which are calculations of the Nsb transmission symbols. Typically, the receiver executes the frequency error estimate to determine the frequency error in the receiver. The frequency error can be due to a difference in the frequencies of the oscillators in the transmitter and receiver, Doppler shift, and so on. Typically, the receiver also performs frame synchronization to detect the start of each frame so that an appropriate sequence of received symbols can be provided for demodulation, deinterleaving and decoding. To support the synchronization of frames, the transmitter usually transmits an orientation sequence through each frame. This orientation sequence contains pilot symbols and is transmitted in designated subbands. The receiver processes the orientation sequence to detect the start of each frame. The Orientation sequence represents the overload that reduces the efficiency of the system. In addition, performance detection based on the orientation sequence is usually not robust, especially in low signal-to-noise (SNR) conditions. Therefore, there is a need for techniques for executing a frequency error estimate and frame synchronization in an OFDM system.

SUMMARY OF THE INVENTION In the present invention, techniques for executing frequency error estimation and frame synchronization in an OFDM system are described. These techniques can provide good performance even in low SNR conditions and are based on a metric that is indicative of the pilot power detected in the receiver. The metric can be defined in several ways, depending on the method used to detect the pilot power. If channel gain calculations are not available, which is typically the case when the frequency error estimate is executed, then the pilot power can be detected (1) by cross-correlating two received symbols obtained in two symbol periods OFDM (typically, two symbols received for two periods of consecutive OFDM symbols), for each of the pilot sub-bands used for the pilot transmission, and (2) accumulating the correlation results for all the pilot sub-bands to obtain a decision statistic. Then the metric is defined based on the decision statistic. For frequency error estimation, a metric value is calculated for each of the multiple hypothesized frequency errors, which are different possible frequency errors in the receiver. The metric value is identified with the largest magnitude among the metric values for the multiple frequency errors hypothesized. The frequency error hypothesized for this identified metric value is provided as an estimated frequency error in the receiver. For frame synchronization, a correlation value for each OFDM symbol period is obtained through the correlation of the identified metric values obtained for N (eg, more recent) OFDM symbol periods with the Nc expected values. The expected values are calculated in a manner consistent with the way in which the metric values are calculated. For example, if the pilot symbols for each pilot sub-band are mixed with a pseudo-random number (PN) sequence by the transmitter, and the values Metrics are obtained by cross-correlation pairs of received symbols, then the expected values are obtained through cross-correlation pairs of chips in the PN sequence. The peak detection is carried out in the correlation values obtained for different periods of OFDM symbol to determine the frame synchronization. Various aspects, embodiments, and features of the invention are described in greater detail below.

BRIEF DESCRIPTION OF THE FIGURES The characteristics and nature of the present invention will be more apparent from the following detailed description when considered in conjunction with the figures in which similar reference characters are identified correspondingly throughout the document and where: Figure 1 shows a transmitter and a receiver in an OFDM system; Figure 2 illustrates the pilot and data transmission for a frame using the frequency-time plane; Figure 3 shows a procedure for recover the data symbols for each frame; Figure 4 illustrates the correlation of Mn metric values with an expected values for frame synchronization; Figure 5 shows a procedure for executing the estimation of integer frequency errors; Figure 6 shows a procedure for executing frame synchronization; Figure 7 shows an OFDM demodulator in the receiver; and Figure 8 shows a specific design for the OFDM demodulator.

DETAILED DESCRIPTION OF THE INVENTION The word "exemplary" is used in the present invention to mean "that it serves as an example, case or illustration". Any modality or design described in the present invention as "exemplary" will not necessarily be construed as preferred or advantageous over other embodiments or designs. Figure 1 shows a block diagram of a transmitter 110 and a receiver 150 in an OFDM system 100. In the transmitter 110, a transmission data processor (TX) 120 receives, formats, and encodes traffic data (ie say, bits of information) to get coded data. The coding increases the reliability of the data transmission and may include error detection coding (eg CRC), advance error correction coding (eg, convolutional, Turbo and / or block) or a combination thereof. Typically, the coding is executed for each data packet, which may have a fixed or variable length. The data processor TX 120 then interleaves the encoded data to obtain the interleaved data. The interleaving provides diversity of time and / or frequency against the detrimental effects of trajectory and can also be executed for each data packet. Next, the data processor TX 120 modulates (ie, symbol maps) the interleaved data based on one or more modulation schemes (eg, QPSK, M-PSK, M-QAM, and so on) to obtain data symbols. The same or different modulation schemes can be used for the data and pilot symbols. An OFDM modulator 130 receives and processes the data and pilot symbols to obtain OFDM symbols. Processing by the OFDM modulator 130 may include: (1) multiplexing the data symbols, pilot symbols, and zero signal values on data sub-bands, pilot sub-bands, and unused sub-bands, respectively, to obtain Nsb transmission symbols for the Nsb sub-bands for each OFDM symbol period, (2) transform the Nsb transmission symbols for each OFDM symbol period with an Nsb-IFFT point to obtain a transformed symbol, and ( 3) append a cyclic prefix to each transformed symbol to form a corresponding OFDM symbol. The pilot symbols can be multiplexed with the data symbols, as described below. The OFDM modulator 130 provides OFDM symbol frames, wherein each frame contains Nsym OFDM symbols and may correspond to a whole number of data packets (e.g., a data packet). A transmitter unit (TMTR) 132 receives and converts the OFDM symbols into one or more analog signals and further conditions (eg, amplifies, filters and overconverts by frequency) the analog signals to generate a convenient modulated signal for transmission over a wireless channel . The modulated signal is then transmitted through an antenna 134 to the receiver 150. In the receiver 150, the transmitted signal is received by an antenna 152 and provided to a receiving unit (RCVR) 154. The receiving unit 154 conditions (eg, filters, amplifies and subverts by frequency) the received signal and digitizes the conditioned signal to obtain a sample stream of entry. An OFDM demodulator 160 receives and processes the input samples to obtain the received symbols. Processing by the OFDM demodulator 160 may include (1) pre-processing the input samples as described below, (2) removing the cyclic prefix added to each received OFDM symbol to obtain a received transformed symbol, and (3) transform each transformed symbol received with an Nsb-FFT point to obtain Nsb symbols received for the Ns sub-bands. The Ns received symbols for each OFDM symbol period includes the data symbols received for the data sub-bands and the pilot symbols received for the pilot sub-bands. The OFDM demodulator 160 also estimates and corrects the frequency error in the receiver, detects the start of each frame, executes the data detection, and provides a sequence of detected data symbols for each frame, as described below. A reception data processor (RX) 170 then demodulates, deinterleaves, and decodes the detected data symbols to provide decoded data. The processing by the OFDM demodulator 160 and the data processor RX 170 is complementary to that performed by the OFDM modulator 130 and the data processor TX 120, respectively, in the transmitter 110. The controllers 140 and 180 direct the operation at transmitter 110 and receiver 150, respectively. Memory units 142 and 182 provide storage for program and data codes used by controllers 140 and 180, respectively. Figure 2 illustrates the data transmission and pilot for a frame in a frequency-time plane 200. The vertical axis of the plane 200 represents the frequency and the horizontal axis represents the time. The Nsb subbands are assigned indexes of 1 to Nsb on the vertical axis. Np subbands are used for pilot transmission, where in general Nsb > Np > _ 1. The pilot sub-bands are indicated by the shaded boxes in Figure 2 and can be distributed (eg uniformly) through the Nsb total sub-bands. The Nsym OFDM symbols for the frame are assigned indexes of 1 to Nsym on the horizontal axis. Each OFDM symbol includes Nsb transmission symbols for the Nsb subbands. In the following description, k is a subband index and n is an index for OFDM symbols and OFDM symbol period. Different OFDM systems can use different values for the various parameters indicated in figure 2. As a specific example, an exemplary OFDM system can have a global system bandwidth of B sys = 6 MHz, use an OFDM symbol with Nsb = 4096 sub-bands, assign Np = 512 sub-bands for pilot, use a cyclic prefix of Ncp = 512 chips, and have a frame length of one second. For this system, each subband has a bandwidth of BWsb = 1.46 KHz (ie 6.0 MHz / 4096), each OFDM symbol has a length of NL = 4608 chips (ie 4096 + 512), each period of The OFDM symbol has a duration of 768 μsec (ie 4608/6, OxlO6), and each frame includes Nsym = 1302 OFDM symbols (ie, 1.0 / 768xl0"6) Figure 2 also shows a pilot multiplexing transmission scheme by frequency division (FDM) where the pilot symbols are transmitted in pilot sub-bands and data symbols are transmitted in sub-bands of data.The pilot sub-bands can be fixed for all OFDM symbol periods or can vary from symbol period to symbol period, frame to frame, and so on.The pilot transmission can also be sent continuously through a whole frame (as shown in figure 2) or it can be sent only in some symbol periods OFDM In any case, the sub-bands used for the pilot transmission and the OFDM symbol periods in which the pilot is transmitted, are known a priori both by the transmitter and by the receiver. For simplicity, the following description assumes that the pilot is continuously transmitted in the designated pilot subbands, as shown in Figure 2. A sequence of NP pilot symbols is transmitted in the NP pilot sub-bands in an OFDM symbol period. The sequence of pilot symbols is denoted as. { p. { k)} and includes a pilot symbol for each pilot sub-band. The same sequence of pilot symbols. { p. { k)} it is transmitted in each of the Nsym symbol periods OFDM for the frame. To facilitate frame synchronization, the pilot symbols for each pilot subband are mixed with a PN sequence. The PN sequence is denoted as. { bn} and contains Nsym PN chips, where each PN chip is either +1 or -1 (ie, bn e. {l, -l.}.). For each pilot subband, the Nsym (same value) pilot symbols for the Nsym symbol periods OFDM for the frame are multiplied with Nsym PN chips to obtain Nsym mixed pilot symbols for that pilot subband. The mixed pilot symbol for each pilot subband of each OFDM symbol period can be expressed as: Pn (.k) = p (k) -b ", for k e P, _ Equation (1) Where Pn. { k) is the mixed pilot symbol for the pilot subband k in the symbol period n; and P is the set of NP pilot sub-bands. p sequences of mixed pilot symbols are obtained for the NP pilot sub-bands based on the NP pilot symbols for these sub-bands and the same sequence PN. The mixed pilot symbols are ultiplexed with the data symbols, processed and transmitted. In the receiver, the symbols received after the FFT can be expressed as: Rn (k) = Sn (k) -Hn (k) -e T + 2 ^ IN ^ + N "(k), Equation (2) Where Sn (k) is the transmission symbol for the subband k in the period of symbols n; Hn (k) is the complex channel gain for the subband k in the period of symbols n, - Nn (k) is the noise for the subband k in the symbol period n; Rn (k) is the symbol received for the subband k in the period of symbols t; ? it is an unknown phase compensation that is constant across all the Nsb subbands; and f is a frequency compensation (in integer number of subbands) to be estimated. The transmission symbol Sn (k) can be a pilot symbol Pn (k) or a data symbol Dn (k). Equation (2) assumes that the fractional frequency error (ie, less than one subband) has been estimated and corrected before executing the FTT. The fractional frequency error up to + B sb / 2 can be estimated based on the cyclic prefix attd to eOFDM symbol or using some other known technique. Fractional frequency error causes interference between bands and is therefore estimated and removed with a phase rotator before executing the FFT, as described below. The frequency error f is a large frequency error that can be caused, for example, by different frequencies of the transmitter and receiver oscillator. The frequency error f is in integer number of subbands because the fractional portion has been corrected before the FFT. The frequency error f results in the transmission symbol Sn (k) sent in the subband k being received in the subband k + f, that is, Sn (k)? Rn (k + f). The entire post-FFT spectrum in the receiver is then changed by f in relation to the pre-IFFT spectrum in the transmitter. The integer frequency error only changes the spectrum and does not cause interference between subbands. This frequency error can then be removed either before or after executing the FFT on the receiver. In the following description, "frequency error" and "frequency compensation" are synonymous terms that are used interchangeably.

Figure 3 shows a flow chart of a method 300 for recovering the transmission symbols Sn (k) for a frame. Initially, the integer frequency error f is estimated based on a metric Mn (f) and the received symbols Rn (k), as described below (step 312). The estimated integer frequency error / is then removed to obtain the corrected frequency symbols Sn (k), which includes the corrected frequency data symbols Dn (k) (ie, received data symbols) for the sub- data bands and the corrected frequency pilot symbols Pn (k) (i.e. pilot symbols received) for the pilot subbands (step 314). Frame synchronization is also executed based on the same metric Mn (f) and the corrected frequency pilot symbols (step 316). Once the integer frequency error correction and frame synchronization is performed, the channel gain Hn (k) can be estimated based on the corrected frequency pilot symbols Pn (k) (step 318). The data detection is then performed on the corrected frequency data symbols Dn. { k) with channel gain calculations ñn. { k) to obtain the detected data symbols £ > n (k), which are calculations of the data symbols Dn (k) sent by the transmitter (step 320). HE provides an appropriate sequence of data symbols detected for the frame for further processing (step 322). Eof the steps in Figure 3 is described in more detail below. For step 312 in Figure 3, the frequency error of integer f is calculated based on the metric Mn (f), which indicates the pilot power detected in the receiver. The metric Mn (f) can be defined in several ways, depending on the methods used to detect the pilot power. The receiver can use different methods to detect the pilot power depending on whether the channel gain estimates are available or not. Various methods of pilot power detection are described below. A cross-correlation method can be used to detect the received pilot power when the channel gain estimates are not available in the receiver. Typically this is the case at the time when the frequency error estimate is made. For this method, the decision statistics for different hypotheses of f can be expressed as: Equation (3) Where, J is a hypothesized frequency error; k + J is a hypothesized subband, which is compensated by / ° e the pilot subband k; Rn (k + J) is the symbol received for the hypothesized subband k + J in the period of symbol n; An. { J) is a decision statistic for the hypothetical frequency error J in the symbol period n; F is a set of frequency errors hypothesized to evaluate, that is, F =. { ?, +1 ... + fma?] Where fmax is the expected maximum frequency error; and "*" denotes the complex conjugate. Each of the frequency errors hypothesized in the set F is a different possible integer frequency error in the receiver. In equation (3), the pilot symbols for the pilot subband k are assumed to be changed by the hypothesized frequency error J, and the received symbols Rn ik + J1) and Rn.x (k + J) for the sub-band. Hypothesized band k + J (instead of the pilot subband k) are used for decision statistics. Equation (3) effectively calculates a cross-correlation between two symbols received for two consecutive OFDM symbol periods, that is, Rn (k + J) -R * l_1 (k + J). This cross-correlation removes the effect of the wireless channel without requiring the channel gain estimate, which is typically not yet available. Equation (3) then accumulates the results of the cross-correlation for all NP pilot sub-bands to obtain the decision statistics An (J) for the hypothetical frequency error J.

The exponential term e ~ j2 ^ NL'N? in equation (3) it is considered for the phase difference (ie, the phase change) between two consecutive OFDM symbols due to the hypothesized frequency error. Different hypothesized frequency errors have different phase changes. Equation (3) also assumes that the wireless channel is approximately constant or varies slowly over two OFDM symbol periods. This assumption is generally true for most systems. The quality of decision statistics An (J) is simply degraded if the wireless channel changes more rapidly. The decision statistic An. { J) is calculated for each of the different hypotheses of f. A set of decision statistics An is obtained. { J), for J < = F for all frequency errors hypothesized in set F.

The metric is defined as: Equation (4) The decision statistic An (J) is usually a complex value and only the real part is used for the metric. The integer frequency error can be estimated as the hypothesized frequency error that results in the maximum magnitude for the metric. This can be expressed as: ? = a? gmax c7 | > Equation (5) where Jn is the estimated integer frequency error determined in the OFDM symbol period n. The metric can have both positive and negative values because the pilot symbols are mixed by the PN sequence. { ccn} . By extracting the magnitude of the metric, the mixing effect is removed. The integer frequency error can be estimated either once using a pair of OFDM symbols or multiple times using multiple pairs of OFDM symbols. Typically, the frequency error varies slowly and the same estimated integer frequency error is gets frequently for each pair of OFDM symbols. Multiple estimates of integer frequency error can be used to detect a bad estimate and to provide greater confidence in the estimated integer frequency error. In any case, an estimated frequency frequency of integer / is obtained for step 312. In addition, the estimation of frequency frequency of integer typically only has to be executed once when the receiver first tunes to the transmitter and there is a large difference between the oscillator frequencies of the transmitter and the receiver. In the correct hypothesis f, the metric Mn (f) can be expressed as: Equation (6) Where vn (k + f) is a noise term for Mn (f) and can be expressed as: Equation (7) Equation (8) In equations (6) and (8), it is the correlation between two PN chips bn and bn-? for two consecutive OFDM symbol periods, where the PN sequence is wrapped. For an additive white Gaussian noise channel (A GN), the channel gain Hn (k + f) can be omitted from equation (6). In this case, the SNR of the metric Mn (f) in the correct hypothesis f can be expressed as: SNRrpjpy ^ 'P? a 2 n, Equation (9) where Ps is the transmission power for each pilot symbol, which is s = E j? A | L where E { x} is the expected value of x; s2 is the noise variance vn (k + f), which is s2 = sn2 - Ps; sn2 is the noise variance? n (k); (Np -Ps) 2 is the signal power of the metric Mn (f) i Np -s2 is the noise power of the Ma (f). SNRfe is the SNR of the metric Mn (f). In equation (9), the Ps / s2 ratio is also the SNR of the received data symbols. If the number of pilot subbands is large enough, then the SNR of the Ma (f) metric can be high even when the SNR of the received data symbols is low. For the exemplary OFDM system described above with NP = 512, the SNR of the metric Mn (f) is approximately 27 dB when the SNR of the received data symbols is 0 dB (ie, SNRfe * 27 dB when Vs / = 0 dB). The integer frequency error can then be estimated reliably based on the metric Mn (f) even in low SNR conditions. In equation (3), the term exponent is used for phase correction due to the hypothesized frequency error. A simplified decision statistic Án (J) can be defined without this phase correction term, in the following way: Equation (10) The metric can then be defined as Mn (J) = An (J). The integer frequency error can be estimated as shown in equation (5). In general 4, (7) is a complex value and the square of the magnitude IA (7) | 2 (e place of magnitude) can be calculated more simply and can be used for the equation (5) . It can be shown that the SNR of the metric Mn (f) defined based on 4 ', (J) is approximately 3 dB worse than the SNR of the metric Mn (f) based on An (J). This 3 dB degradation in SNR can be compensated by doubling the number of pilot subbands. An adjusted filter method can be used to detect the received pilot power when channel gain calculations are available at the receiver. For this method, the decision statistic can be defined as: Equation (11) Where Ñn (k + J) is the estimated channel gain for the hypothesized subband k + J. In the equation (11), multiplication by ñn * (k + J) removes the effect of the wireless channel, and multiplication by P * (k) removes the modulation in the pilot symbol. The metric Mn (f) can then be defined to be equal to the real part of the decision statistic An "(J), that is, , similar to that shown in equation (4). Other methods can also be used to detect the received pilot power. The metric is defined based on the decision statistics provided by these methods. For step 314 in Figure 3, the estimated integer frequency error J is removed to obtain the corrected frequency symbols Sn (k). The correction of the integer frequency error can be executed either before or after the FFT on the receiver. For post-FFT frequency error correction, the received symbols Rp (k) are simply translated by the subbands J, and the corrected frequency symbols Sn (k) are obtained as Sn (k) = Rn (k + J), for all applicable values of k. For the correction of pre-FFT frequency error, the estimated integer frequency error J can be combined with the fractional frequency error to obtain the total frequency error. The input samples are then rotated in phase by the total frequency error, and the FFT is executed in the rotated phase samples. The oscillator frequency of the receiver can also be adjusted by means of a locked phase loop (PLL) to correct the estimated frequency error J. For step 316 in Figure 3, the frame synchronization is executed based on (1) the same metric Mn (f) used for frequency error estimation (2) the corrected frequency pilot symbols Pn (k). The frequency error estimate in step 312 provides the maximum value of metric Ma for each OFDM symbol period n, which can be expressed as: Mn = Mn (J), Equation (12) where Mn (J) can be defined based on An (J) or An (J).

The simplified decision statistic It can use if the integer frequency error is corrected before executing the FFT. The metric values Mn are obtained based on the corrected frequency pilot symbols by means of the frequency error estimation. A cross-correlation between the values Mn and an is performed for each OFDM symbol period, as follows: Nr- \ Cn =? Mn_t • a N.-i, f = 0 Equation (13) where Nc is the length of the correlation, which is NL > Nc > l; and Cn is the result of the cross-relation between (1) the Mn values for the most recent NcD symbol period periods and (2) the an values for the first Nc OFDM symbol periods in each frame. Figure 4 illustrates the correlation between the values Mn and an. A truncated sequence with the first Nc values an for a frame is shown in the upper part of figure 4 and is given indexes of 1 to -Nc. To Half of Figure 4 shows a sequence with the Nc + 1 most recent Mn values and is given indices of n-Nc to n. For each OFDM symbol period n, a correlation value Cn is obtained by means of the correlation of the truncated sequence with the sequence Mn for the period of OFDM symbols. The sequence Mn changes effectively to the left when a new Mn value is obtained for the next OFDM symbol period. The sequence an remains stationary. The an values are the expected values for the Mn values. For the modality described above, the values an are defined as an = bn 'bn-? because the Mn values are obtained through the correlation of two consecutive received pilot symbols that are mixed with two PN ba and bn-? . For this mode, improved performance for frame synchronization can be achieved if the PN sequence. { bn} is defined for that sequence. { an } It is also a PN sequence. Very particularly, the cross-correlation between the sequence. { to } And its changed versions should be zero or low, except when the two sequences are aligned. For the modality in which the Mn values are obtained based on the decision statistics shown in the equation (11), the values aa are simply equal to the values bn for the PN sequence. In general, the values n they depend on the way in which the Mn values are obtained. Peak detection is performed on the correlation values Cp obtained for different periods of OFDM symbol to determine the start of a frame. A correlation peak appears when the Mn values are aligned with the values an. Peak detection can be performed in several ways. For example, the correlation value Cn for each OFDM symbol period can be compared against a threshold value, and a correlation peak can be declared provided that the correlation value exceeds the threshold value. As another example, a correlation peak can be declared as long as the correlation value Cn exceeds the average or the next highest correlation value by a certain amount. Frame synchronization can also be performed to detect the end of a frame or some other part of the frame. This can be achieved by selecting different portions of the sequence of values an corresponding to the part of the frame to be detected. In general, the correlation is between (1) Mn values for Nc OFDM symbol periods "marked" by the current OFDM symbol period ny (2) an expected values for the Mn values in a designated OFDM symbol period or portion of the plot. For an AGN channel, the correlation between Mn and aa provides a gain Nc (the length of the correlation) in the SNR of the correlation value Cn in the peak. Therefore, robust detection for frame synchronization is possible even under poor SNR conditions. The correlation length Nc can be selected based on several factors. A larger value for Nc provides greater SNR gain and greater reliability in frame detection. However, more memory is needed to store the Mn values for the larger value of Nc. To simplify the processing of frame synchronization, the Mn values can be quantized to L bits, where L >;1. For example, the values Mn can be quantified to a bit by executing difficult decisions in these values. The quantized Mn values (denoted as Ñn) can be correlated with the values an as shown in equation (13). If the pilot symbols are mixed with the PN sequence, as described above, then the pilot symbols can not be recovered until the frame synchronization has been executed and the beginning of the frame is known. The corrected frequency pilot symbols Pn (k) can then be decoded by multiplying these symbols with the complex conjugate of the PN sequence. The channel gain Hn (k) can be estimate based on the decoded pilot symbols. For step 320 in figure 3, the data detection is performed on the corrected frequency data symbols Dn (k), as follows: Equation (14) where ñn (k) is the channel gain estimate for subband J in the period of symbol n; and iW is the data symbols detected for the subband k in the period of symbol n. The detection of data can also be performed in other ways, as is known in the art. The data symbols detected for the frame are provided as a sequence of the subsequent processing. Figure 5 shows a flow chart of a method 500 for executing integer frequency error estimation in the receiver in the OFDM system. The method 500 can be used for step 312 in FIG. 3. Initially, a value for the metric Mn (f) is calculated for each of a number of frequency errors hypothesized based on the received symbols (block 510). This can be achieved by selecting an error of frequency hypothesized J for evaluation (step 512). For each pilot subband k, a cross-correlation is made between two received symbols obtained in two consecutive OFDM symbol periods in a hypothesized subband k + J which is compensated by J of the pilot subband J (step 514). It may or may not include a phase correction term in the cross correlation, as shown in equations (3) and (10). The results of the cross-correlation for all the pilot sub-bands are accumulated to obtain a decision statistic An (J) or A ^ iJ) for the hypothetical frequency error J (step 516). If all the frequency errors hypothesized have not been evaluated (as determined in step 518), then the procedure returns to step 512 to select another hypothesized frequency error for evaluation. Otherwise, a set of metric values is obtained from a set of decision statistics obtained for all the hypothesized frequency errors that have been evaluated (step 520). The metric can be the real part of the decision statistic or all the decision statistics. The frequency error is then estimated based on the set of metric values (block 530). This is achieved by calculating the magnitude (or the square of the magnitude) of each metric value. The metric value is identified in the set with the largest magnitude (or the largest square quantity) (step (532).) The frequency error hypothesized for this identified metric value is provided as the estimated integer frequency error (step 534). The estimation of integer frequency error typically only needs to be executed once, for example, when the receiver is first tuned to the transmitter or, at the start of a data transmission after a prolonged period of inactivity. Therefore, the mechanism used to calculate and track the fractional frequency error can be used to keep the frequency blocked in the receiver Figure 6 shows a flow chart of a method 600 for executing frame synchronization in the receiver in the OFDM system The method 600 can be used for step 316 in figure 3. Initially, a metric value Mn is calculated for each OF symbol period DM based on the cross-correlation between two received symbols obtained in two consecutive OFDM symbol periods in each pilot subband, as described above (step 612). The metric value Ma is obtained after the frequency error of integer f has been calculated and removed either before FFT or after FFT. For each OFDM symbol period, a sequence of Mn values for Nc OFDM symbol periods (eg, more recent) is correlated with a sequence of values an to obtain a correlation value Cn for the OFDM symbol period, such as shows in equation (13) (step 614). The values an are the expected values for the Mp values to the appropriate time alignment. The peak detection is then executed at the correlation values obtained for different periods of OFDM symbol (step 616). Frame synchronization is declared when a correlation peak is detected (step 618). The detected correlation peak may correspond to the beginning of a frame or some other part of the frame, depending on the sequence of values still used for the correlation. The frame synchronization can be performed on a continuous basis, for example, for each frame. Frame synchronization can also be performed as needed, for example, at the start of each data burst. Figure 7 shows a block diagram of an OFDM demodulator 160 mode at receiver 150 in Figure 1. A pre-processor 710 receives and processes input samples from receiver unit 154 and provides pre-processed samples. The pre-processor 710 can perform a sample rate conversion, possibly a whole and fractional frequency correction, the removal of the cyclic prefix, and so on, as described below. An FFT unit 720 executes an FFT on the pre-processed samples for each received OFDM symbol to obtain the received symbols Rn (k). A metric calculation unit / frequency error estimator 750 estimates the integer frequency error at the receiver 150 based on the metric Mn (f) and the received symbols Rn (k), as described above. The unit 750 provides the estimated integer frequency error J either to the preprocessor 710 or a frequency correction unit 730. The preprocessor 710 can perform the frequency correction of integer frequency to FFT, and the correction unit of frequency 730 can execute the integer frequency correction after FFT. A frame synchronization unit 760 receives Mn metric values from the metric calculation unit 750, executes frame synchronization based on these metric values, and provides a frame synchronized signal to a channel estimator 770. The synchronized signal of Frames indicates the start of each frame. The frequency correction unit 730 provides corrected frequency data symbols > "(K) to a data detector 740 and corrected frequency pilot symbols Pn (k) to the channel estimator 770. The channel estimator 770 decodes the corrected frequency pilot symbols based on the frame synchronization signal, calculates the channel gain based on the decoded pilot symbols, and provides channel gain estimates Ñn (k) to the data detector 740. The data detector 740 performs data detection on the corrected frequency data symbols with the channel gain estimates as shown in equation (14) and provides detected data symbols í > n (k). Figure 8 shows a block diagram of a specific design for the OFDM demodulator 160. Within the pre-processor 710, a sample rate converter 810 receives and converts the input samples (at the sampling rate) into interpolated samples ( at the speed of chips). The chip rate refers to the speed of the chips that constitute the OFDM symbols in the transmitter. The sampling rate refers to the speed used by the receiving unit 154 to digitize the received signal. The sampling rate is typically selected to be greater than the chip rate to simplify filtering in the receiver. A time acquisition unit 812 acquires the timing of the OFDM symbols received (by example, based on the cyclic prefix), determines the boundaries of the received OFDM symbols, and provides timing signals to other processing units within the OFDM 160 demodulator (which is not shown in Figure 8 for simplicity). A fractional frequency error detector 814 calculates the fractional frequency error in the receiver based on the cyclic prefix in the interpolated samples. A phase rotator 816 applies the fractional frequency error correction to the interpolated samples and provides corrected frequency samples. A cyclic prefix removal unit 818 removes the cyclic prefix attached to each OFDM symbol by the transmitter and provides the pre-processed samples. For the modality shown in figure 8, the unit of calculation of metric / estimator of error of frequency 750 uses the metric defined on the basis of the method of cross-correlation. Within unit 750, a correlator 850 executes cross-correlation in pairs of received symbols obtained in two consecutive OFDM symbol periods in a hypothesized sub-band k + 7- For each hypothetical frequency error J, the cross-correlation is executed for each of the pilot sub-bands and may or may not take into account the phase correction of the hypothesized frequency error J. An accumulator / post-processing unit 852 accumulates the results of correlation for all subbands for each hypothesized frequency error to obtain a decision statistic An (J) for that hypothesis. Unit 852 provides a metric value Mn J) for each hypothesized frequency error based on the real part of decision statistics An (J) or the complete decision statistic Án (J).

The correlator 850 and the accumulator 852 form the metric calculation unit. A magnitude detector 854 detects the metric value Mn (J) with the largest magnitude for each OFDM symbol period. The detector 854 provides (1) the estimated frequency error J to the frequency correction unit 730 or the fractional frequency error detector 814 and (2) the metric values Mn to the frame synchronization unit 760. For the mode shown in FIG. 8, a correlator 860 within the frame synchronization unit 760 correlates the metric values Mn with the values an and provides a correlation value Cn for each OFDM symbol period. A peak detector 862 executes peak detection in the correlation values Cn for different periods of OFDM symbol and provides the frame synchronization signal. For clarity, both frequency error estimation and frame synchronization have been described for an exemplary OFDM system. In general, the frequency error estimation techniques described above can be used independently of frame synchronization. In addition, the frame synchronization techniques described above can be used independently of the frequency error estimate, which can be achieved in several ways. The frequency error estimation techniques, or the frame synchronization techniques, or both the frequency error estimation techniques and the frame synchronization techniques described herein can be used in the receiver, depending on their design. The pilot transmission scheme described above supports both frequency error estimation and frame synchronization. Other pilot transmission schemes can also be used. For example, the pilot symbols may be transmitted in a non-continuous fashion (ie, only in periods of designated OFDM symbols), in different sub-bands in different periods of OFDM symbols, and so on. The pilot symbols do not have to be mixed with the PN sequence for the frequency error estimation. The metric is defined in a manner corresponding to and consistent with the pilot transmission scheme employed by the OFDM system. Frequency error estimation techniques and the frame synchronization techniques described in the present invention can be executed through various means. For example, these techniques can be executed in hardware, software, or a combination thereof. For a hardware execution, the processing units used to execute frequency error estimation and / or frame synchronization can be executed within one or more specific application integrated circuits (ASIC), digital signal processors (DSP) , digital signal processing devices (DSPD), programmable logic devices (PLD), field programmable gate (FPGA) arrangements, processors, controllers, microcontrollers, microprocessors, other electronic units designed to perform the functions described in present invention, or a combination thereof. For software execution, frequency error estimation and frame synchronization techniques can be executed with modules (for example, procedures, functions and so on) executing the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit 182 in Figure 1) and executed through a processor (e.g., controller 180). The memory unit can be run inside the processor or outside of the processor, in which case it can be communicatively coupled to the processor through various means, as is known in the art. The prior description of the described embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not intended to be limited to the modalities shown herein but will be accorded the broadest scope consistent with the principles and novel features described herein.

Claims (30)

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as a priority: CLAIMS

1. - A method for executing frequency error estimation and frame synchronization in a receiver in an orthogonal frequency division multiplexing (OFDM) communication system, the method comprises: estimating the frequency error in the receiver based on received symbols and a metric indicative of the pilot power detected; withdraw the estimated frequency error to obtain the corrected frequency pilot symbols; and execute the frame synchronization based on the metric and the corrected frequency pilot symbols.

2 - The method according to claim 1, characterized in that the metric is based on the cross-correlation between two symbols received for two periods of symbols.

3.- The method of compliance with the claim 1, characterized in that the metric is in symbols received from filtering adjusted with channel gain estimates.

4. - The method according to claim 1, characterized in that the estimation of the frequency error includes: calculating, for each of a plurality of hypothesized frequency errors, a value for the metric based on the received symbols, wherein each one of the hypothesized frequency errors corresponds to a possible error of different frequency in the receiver, and where a plurality of metric values are obtained for the plurality of frequency errors hypothesized; identify a metric value with the greatest magnitude among the plurality of metric values; and provide the hypothesized frequency error for the metric value identified as the estimated frequency error.

5. - The method according to claim 1, characterized in that the execution of the frame synchronization includes: calculating a value of the metric for a current symbol period based on the corrected frequency pilot symbols obtained in one or more periods of symbol, including the current symbol period; correlating a plurality of metric values, obtained for a plurality of symbol periods marked by the current symbol period, with a plurality of expected values to obtain a correlation value for the current symbol period, wherein the plurality of expected values are expected values for the plurality of metric values in a designated symbol period, and execute the peak detection in the correlation values obtained for different symbol periods to determine the frame synchronization.

6. - The method according to claim 1, further comprising: decoding the pilot symbols of corrected frequency with a sequence of pseudo-random numbers (PN) to obtain decoded pilot symbols, wherein the PN sequence is aligned based on frame synchronization; and estimate the channel gain based on the decoded pilot symbols.

7. - The method according to claim 6, further comprising: executing the data detection in the corrected frequency data symbols with channel gain estimates to obtain detected data symbols.

8. - The method according to claim 1, characterized in that the estimated frequency error is removed through the rotation of the time domain samples before executing the Fast Fourier Transform (FFT) to obtain the received symbols.

9. - The method according to claim 1, characterized in that the estimated frequency error is removed by changing the subband rates by means of the estimated frequency error.

10. A receiver apparatus in an orthogonal frequency division multiplexing (OFDM) communication system, comprising: a frequency error estimator for estimating the frequency error in the receiver based on a metric and the symbols received, where the metric is indicative of the pilot power detected; an operating frequency correction unit for withdrawing the estimated frequency error to obtain the corrected frequency pilot symbols; and an operational frame synchronization unit for executing frame synchronization based on the metric and pilot symbols of corrected frequency.

11. - A receiving device in a frequency division multiplexing communication system orthogonal (OFDM), comprising: means for estimating the frequency error in the receiving apparatus based on a metric and received symbols, wherein the metric is indicative of the detected pilot power; means for withdrawing the estimated frequency error to obtain the corrected frequency pilot symbols; and means for executing frame synchronization based on the metric and pilot symbols of corrected frequency.

12.- A method for executing frequency error estimation in a receiver in an orthogonal frequency division multiplexing (OFDM) communication system, the method comprising: calculating, for each of a plurality of hypothesized frequency errors, a value for a metric based on received symbols, where the metric is indicative of the detected pilot power, where each of the frequency errors hypothesized corresponds to a possible error of different frequency in the receiver, and where a plurality of metric values for the plurality of hypothesized frequency errors; and estimate the frequency error in the receiver with based on the plurality of metric values.

13. - The method according to claim 12, characterized in that the metric is defined based on the cross-correlation between two symbols received for two periods of symbols.

14. - The method according to claim 13, characterized in that the metric value for each hypothesized frequency error is calculated by: calculating, for each of a plurality of pilot sub-bands used for pilot transmission, the correlation crossed between two received symbols obtained in two symbol periods for a hypothesized sub-band that is compensated by the hypothesized frequency error of the pilot subband, add the results of the cross-correlation for the plurality of pilot sub-bands to obtain a decision statistic, and derive the metric value for the hypothetical frequency error based on the decision statistic.

15. - The method according to claim 13, characterized in that the cross-correlation between the two symbols received for a hypothetical frequency error considers the phase difference between the two symbols received due to the hypothesized frequency error.

16. - The method according to claim 12, wherein the metric is defined based on adjusted filtered symbols.

17. - The method according to claim 16, characterized in that the metric value for each hypothesized frequency error is calculated by: multiplying, for each of a plurality of pilot sub-bands used for the pilot transmission, an estimated of channel gain for a hypothesized sub-band with a symbol received for the hypothesized sub-band to obtain a filtered symbol adjusted for the pilot sub-band, the hypothesized sub-band is deviated from the pilot sub-band by the error of hypothetized frequency, add the filtered symbols adjusted for the plurality of pilot sub-bands to obtain a decision statistic, and derive the metric value for the hypothesized frequency error based on the decision statistic.

18. The method according to claim 12, characterized in that the estimation of the Frequency error includes: identifying the metric value with the largest magnitude among the plurality of metric values, and providing a hypothesized frequency error for the metric value identified as an estimated frequency error for the receiver.

19. - A receiving apparatus in an orthogonal frequency division multiplexing communication system (OFDM), comprising: an operational correlation unit for calculating, for each of a plurality of hypothesized frequency errors, a value for a metric based on received symbols, where the metric is indicative of the pilot power detected, where each of the frequency errors hypothesized corresponds to a possible error of different frequency in the receiver, and where a plurality of metric values is obtained for the plurality of frequency errors hypothetized; and an operating detector for estimating the frequency error in the receiving apparatus based on the plurality of metric values.

20. The receiver apparatus according to claim 19, characterized in that the correlation unit is operative, for each frequency error hypothesized, for: calculating, for each of a plurality of pilot sub-bands used for the pilot transmission, the cross-correlation between two received symbols obtained in two symbol periods for a hypothesized sub-band that is compensated by the hypothesized frequency error of the sub - pilot band, add the results of the cross-correlation for the plurality of pilot sub-bands to obtain a decision statistic, and derive the metric value for the hypothesized frequency error based on the decision statistic.

21. A receiver apparatus in an orthogonal frequency division multiplexing communication system (OFDM), comprising: means for calculating, for each of a plurality of hypothesized frequency errors, a value for a metric based on received symbols , wherein the metric is indicative of the detected pilot power, wherein each of the frequency errors hypothesized corresponds to a possible error of different frequency in the receiving apparatus, and where a plurality of metric values for the plurality of frequency errors hypothesized; and means to estimate the frequency error in the receiving apparatus based on the plurality of metric values.

22. A processor-readable medium for storing operable instructions for: calculating, for each of a plurality of hypothesized frequency errors, a value for a metric based on received symbols, wherein the metric is indicative of the detected pilot power, wherein each of the hypothetized frequency errors corresponds to a possible error of different frequency in the receiver, and wherein a plurality of metric values for the plurality of frequency errors hypothesized is obtained; and estimating the frequency error in the receiver based on the plurality of metric values.

23. A method for executing frame synchronization in a receiver in an orthogonal frequency division multiplex communication (OFDM) communication system, the method comprising: calculating a value for a metric for a current symbol period based on the pilot symbols received for one or more symbol periods including the current symbol period, wherein the metric is indicative of the detected pilot power; correlate a plurality of metric values, obtained for a plurality of symbol periods marked by the current symbol period, with a plurality of values expected to obtain a correlation value for the current symbol period, wherein the plurality of expected values are expected values for the plurality of values metrics in a designated symbol period; and executing the peak detection in correlation values obtained for different symbol periods to determine the frame synchronization.

24. - The method according to claim 23, further comprising: executing the frequency error estimate to obtain an estimated frequency error at the receiver, and wherein the metric value for the current symbol period is considered for the estimated frequency error.

25. - The method according to claim 23, characterized in that the peak detection is performed through: comparing the correlation value for the current symbol period against a threshold value; and declaring the frame synchronization if the correlation value is greater than the threshold value.

26.- The method of compliance with the claim 23, characterized in that the metric value for the current symbol period is obtained based on the cross-correlation between the pilot symbols received for the current symbol period and the pilot symbols received for the previous symbol period.

27. The method according to claim 23, characterized in that for each of a plurality of pilot sub-bands used for the pilot transmission, the pilot symbols for the pilot sub-band are mixed with a sequence of pseudo-random number. (PN) before the transmission.

28. The method according to claim 27, characterized in that each of the plurality of expected values is obtained by cross-correlating a respective pair of chips in the sequence PN.

29. A receiving apparatus in an orthogonal frequency division multiplexing (OFDM) communication system, comprising: a metric operational calculation unit for calculating a value for a metric for a current symbol period based on the pilot symbols received for one or more symbol periods including the current symbol period, wherein the metric is indicative of the detected pilot power; an operational correlator to correlate a plurality of metric values, obtained for a plurality of symbol periods marked by the current symbol period, with a plurality of expected values to obtain a correlation value for the current symbol period, wherein the plurality of expected values are expected values for the plurality of metric values in a designated symbol period; and an operational peak detector for executing peak detection in correlation values obtained for different symbol periods to determine frame synchronization.

30. The receiving apparatus according to claim 29, characterized in that for each of the plurality of pilot sub-bands used for the pilot transmission, pilot symbols for the pilot subband are mixed with a sequence of pseudo-random numbers. (PN) before the transmission. 31.- The receiving device according to claim 30, characterized in that the metric value for the current symbol period is obtained based on the cross-correlation between the pilot symbols received for the current symbol period and the pilot symbols received for a previous symbol period, and where each of the plurality of expected values is obtained cross-correlating a respective pair of chips in the PN sequence. 32.- A receiving apparatus in an orthogonal frequency division multiplexing communication system (OFDM), comprising: means for calculating a value for a metric for a current symbol period based on the pilot symbols received for one or more symbol periods including the current symbol period, where the metric is indicative of the detected pilot power; means for correlating a plurality of metric values, obtained for a plurality of symbol periods marked by the current symbol period, with a plurality of expected values to obtain a correlation value for the current symbol period, wherein the plurality of values expected are expected values for the plurality of metric values in a designated symbol period; and means for executing peak detection in correlation values obtained for different symbol periods to determine frame synchronization.